Alunite-jarosite minerals

Alunite-jarosite minerals (and the other secondary sulfates) form, in part, because of evaporative concentration of pore and capillary waters in pyritic materials, and also at depth in saturated tailings (Dubrovsky et al. 1985). They are found in soils beneath acid sulfate evaporation ponds (Peterson et al. 1983), and are also precipitated directly from acid mine waters (cf. Filipek et al. 1987 Alpers et al. 1989). The alunite-jarosite mineral group also occurs in the weathered zones on top of metal sulhde deposits (Scott 1987) and in the sediments of acid hypersaline lakes (Alpers et al. 1992). 

Twelvefold coordination of lower valence actinides is inferred only for the orthophosphate mineral eylettersite, (Th,Pb)i.xAl3(P04,Si04)2(0H)6 [77], which is a member of the alunite-jarosite supergroup that includes the isotypic florencite group of phosphates and arsenates. Charge balance in eylettersite is maintained through partial occupancy of the Th site, and by substitution of Si forP. 

Scorr, K. M. 1987. Solid solution in, and classification of, gossan-derived members ol the alunite-jarosite family, northwest Queensland, Australia, Am. Mineral. 72 178-87,... 

Sulfate Minerals (Figure 6.6) Fifteen sulfate minerals are present in the thermodynamic database. The calculations show that many are supersaturated in our water samples. Several members of the alunite-jarosite group are grossly supersaturated, but alunite and jarosite are known to be supersaturated without precipitating in some acidic surface waters (Alpers et al., 1994). Aluminum hydroxy sulfates, Al(OH)SC>4 (jurbanite), and Al4(OH)ioSQ4 (basaluminite), are also supersaturated at some points... 

The stabilities of alunite, jarosite, gibbsite, goethite, and K-feldspar as functions of activities of Fe, Al, H, and SO in solution are shown in Figure 1. Alunite is a soluble mineral and is typically found in a low pH, high SO, and Fe-poor environment (6). When the concentrations of dissolved SO, Al, and K exceed 10 m, alunite is more stable than gibbsite below pH 3. Alunite has been observed in some uranium tailings impoundments in New Mexico... 

Minerals of alunite-jarosite groups have the general formula MAsCSOJjCOH) where M is H3O+, NH4+, Na+, K+, Rb+, Ag+, T1+, O.SPb " and A is AP" (alunite) or Fe (jarosite). Related synthetic materials MFe(S04)2(0H)6 (M = H3O+, NH4+), MGa3(S04)2(0H)6 also form solid solutions Hydronium alunite can be prepared by hydrothermal reaction from Al2(S04)3.16H2O at 250 °C while only the mixed derivative is obtained by refluxing at 105 °C. Sulphate... 

The mineral alunite, and its iron equivalent jarosite, is a special case. Alunite (KAl2(S02)2(OH)2) contains four sites where elements containing stable isotopes are found and both the sulfate and hydroxyl anionic groups may provide information on fluid source and condition of formation. 

Menchetti, S. Sabelli, C. (1976) Crystal chemistry of the alunite series Crystal structure refinement of alunite and synthetic jarosite. Neues Jahrb. Mineral. Mh, pp. 406-17. 

More common minerals in such a setting are jarosite and alunite, which often dissolve incongruently to form their oxyhydroxides, releasing protons. The jarosite reaction produces significant acidity, whereas alunite is a very weak acid. For the reaction... 

During chemical oxidation of uranium ore and gangue minerals in the presence of sulfuric acid and excess free oxygen, the precipitation of jarosite and alunite is indicated by the reactions... 

Figure 1. Activity diagram of jarosite-alunite-potassium feldspar-gibbsite-goethite system at 1 bar and 298 K. Modified from Bladh (18). Tailings pore water from Grants Mineral Belt, New Mexico and Maybell, Colorado are denoted by O and, respectively. Median ground water composition (19) is represented by. ...

These results show how geochemical modeling can be used with mineralogical studies to evaluate chemical reactions occurring in tailings pore water and subjacent soils. The identification of gypsum, jarosite, and alunite support the predictions made by PHREEQE that these secondary minerals precipitated from tailings pore water. 

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